The present invention relates to a developing device for a facsimile apparatus, copier, printer or similar electrophotographic image forming apparatus.
Many of modern image forming apparatuses of the type described have a developing device operable with a toner, or one component type developer, having high electric resistance. This kind of developer basically eliminates the need for the maintenance of the developing device and reduces the size of the device. Some different types of developing devices have heretofore been proposed, as follows. A first type of developing device has a developing roller having an elastic surface and contacting a photoconductive drum, and a blade contacting the surface of the roller to form a thin uniform toner layer thereon. A bias voltage for development is applied to the developing roller. A second type of developing device has a developing roller having a hard surface and contacting a photoconductive belt, and a blade contacting the surface of the roller to form a thin uniform toner layer thereon; a bias voltage is also applied to the developing roller. A third type of developing device has first conveying means corresponding to a developing roller, and second conveying means intervening between the first conveying means and a photoconductive element and implemented as a belt, as disclosed in Japanese Patent Laid-Open Publication (Kokai) No. 61-34557. In this type of developing device, a toner is transferred from the first conveying means to the photoconductive element via the second conveying means or belt.
The first type of developing device described above have some problems since the surface of the developing roller is elastic, as follows. (1) It is difficult for the blade to form a thin uniform toner layer on the developing roller. (2) The surface of the roller creeps and fails to contact the photoconductive drum and blade uniformly, resulting in defective development. (3) It is difficult to charge the toner uniformly, i.e., toner particles charged to polarity opposite to desired polarity appear, contaminating the background of an image.
The second type of developing device can eliminate the above-stated problems (1) and (2), but it cannot eliminate the problem (3). Moreover, this type of device brings about another problem that (4) it needs a mechanism for driving the photoconductive belt (drive roller, gears, etc.) and a mechanism for balancing the tension of the belt, increasing the cost.
Even the third type of developing device cannot eliminate the above problem (4) although it can eliminate the problem (3).